WASHINGTON — With the clock running down on Obamacare enrollment, the administration sought to persuade young people to sign up for health coverage on Tuesday by telling them how much it hurts not be insured — that is, how much it can hurt the wallet.

Take the humble ankle sprain, one of the most common injuries among young adults under the age of 25. Treating it can cost $2,290. Then there’s the broken arm: On average $7,700. And people without health insurance get to pay full freight.

Or as U.S. Health and Human Services Secretary Kathleen Sebelius put it in a government blog: “This can be a huge financial blow to young people and families alike.”

That is the message the administration hopes will be heard by college-age kids and others who do not have health insurance, but could qualify for federal subsidies to help purchase coverage. Some could also qualify for the Medicaid health program for the poor.

Open enrollment ends March 31.

In a promotion aimed at fans of the annual college basketball playoff series known as March Madness, Sebelius’ Department of Health and Human Services and the President’s Council on Fitness, Sports and Nutrition released data looking at the economic costs of common sports injuries like sprains and fractures — just the sort of thing to send a young person to the emergency room.

Young people are vital to the success of President Barack Obama’s signature healthcare law. Obamacare prevents insurance companies from penalizing people who are sick or older. And so the new marketplaces need young people who are cheaper to insure to make up for the higher financial risks posed by others.

But so far, the administration’s target audience of people aged 18 to 34 have not been signing up in such large numbers, a trend that could lead to higher insurance costs down the road if it continues.

More than 5 million people have enrolled in private health insurance under Obamacare, according to the administration. But the latest breakdown shows the number of younger adults stuck at 25 percent of the enrollment population, versus the 38 percent target that the administration laid out before last October’s botched rollout.

Administration officials say younger people could sign up in huge numbers in the final days of the open enrollment period.